1. Technical Field
The present invention relates to an image forming apparatuses.
2. Background Art
Image forming apparatuses employing electrophotography, for example, printers, are used to form images with toner. Such an image forming apparatus forms an image by: developing a latent electrostatic image formed on an image bearing member with toner; transferring the thus-obtained toner image to a recording medium; and fixing the toner image thereon by melting the toner upon application of heat. This fixing process requires a lot of electric power to melt and fuse toner. For this reason, using toner having a low temperature fixability is an issue in terms of energy efficiency.
In efforts to improve this low temperature fixability of toner, for example, JP-2010-077419-A and JP-2010-151996-A disclose toner containing a crystalline resin as a binder resin.
However, as the content of such a crystalline resin increases in toner containing the crystalline resin as a binder resin, the toner becomes soft because the resin is soft. Such toner is weak to stirring stress in a development unit so that toner and carrier are easily agglomerated, resulting in production of defective images.
The hardness of toner can be improved by, for example, introducing a urethane bond, etc. into a crystalline resin.
However, since the toner becomes hard, it loses ductility. For this reason, anchoring between the toner and a recording medium is lowered, thus degrading the low temperature fixability of the toner. When an image is formed in monochrome mode or a half tone image is formed, the attachment amount of toner is few in particular. In such a case, the attachment force between toner is not strong. As a consequence, the low temperature fixability is worsened in comparison with when forming an image with a large amount of toner, for example, forming an image in color mode.
Typically, increasing the surface pressure (pressure of the contact surface) of the nip (nipping portion) of a fixing unit is a way to improve anchoring between toner having low ductility and a recording medium. However, the releasability between toner and a fixing member is lowered, thereby degrading the hot offset resistance of the toner. In addition, to maintain durability, the substrate of a fixing roller and a core material of a pressure roller are thickened, which leads to an increase of the heat capacity of such fixing members. This is not preferable in terms of energy efficiency.